This invention relates to battery chargers and in particular to battery charger circuits for lead-acid cell batteries. Various charging modes for battery chargers are known in the prior art. In the constant current mode, for example, a constant current level is passed through the battery cells either for a fixed time or until the battery is determined to be fully charged. A variation of the constant current mode is to pass a large current through the battery until the battery reaches a predetermined state of charge, and then reduce the current level for completing the charge. A third mode is the decreasing current charge rate mode in which the charging current is continuously reduced from a large initial value toward zero in proportion to the remaining amount of charge to be stored. In the constant potential mode, a fixed potential is applied across the battery and held until charge is completed. Current limiting means must, however, be used to prevent an excessive initial surge of current into a fully discharged battery. Such an initial current surge could harm battery cells and also the series controlling element of the charger.
Most prior art battery chargers are pre-set for a nominal room temperature of approximately 75.degree. F. for charging purposes, however, batteries generally require a higher charging voltage in cold temperatures than in warm temperatures.
Prior art battery chargers also generally require a relatively constant input voltage to limit dissipation in the series controlling element of the charger.